The invention relates to a laser for the generation of narrow band radiation of high spectral purity comprising a laser resonator which includes two reflective elements and a laser active medium placed between them, at least one wavelength selective element and a wavelength selective outcoupling device for outcoupling the radiation from the laser resonator.
Such a laser is known from U.S. Pat. No. 5,856,991 (Ershov). The excimer laser described therein comprises a grid placed in the resonator as the wavelength selective element and an etalon as the wavelength selective outcoupling device for coupling out the radiation from the laser resonator.
The invention is concerned with the problem of with a laser of that kind generating highly stable narrow band radiation as with a band width narrow as possible. This is especially important in photolithographical applications for the production of integrated circuits. For this purpose wavelengths of  less than 250 nm are required to create structures in dimensions  less than 0,25 xcexcm (for structures in dimensions  less than 0, 18 xcexcm wavelengths of  less than 200 nm are required). In such spectral ranges achromatically imaging optics can hardly be produced. Therefore, the radiation used must be very narrow banded in order to keep imaging distortions caused by chromatic aberrations to a minimum. In the field of photolithography, which is a particular concern of this invention, band widths in the range of  less than 0,6 pm are acceptable for refractive imaging optics.
In such applications of laser radiation another important radiation property is the so-called spectral purity (also called xe2x80x9c95% integralxe2x80x9d). The spectral purity of the radiation is indicated, for example, by that wavelength interval within which 95% of the total pulse energy lies.
In the state of the art varied attempts to improve both, the band width and the spectral purity are known.
One such attempt for the improvement of spectral purity is the realization that the so-called wave front curvature has a substantial influence on the band width and especially on the spectral purity of the laser radiation released. The German Gebrauchsmuster (petty patent) 298 22 090 describes lasers in which such a wave front curvature is corrected.
A further attempt for the improvement of the band width and the spectral purity is pursued in the U.S. Pat. No. 5,150,370 (Furuya et al.) and U.S. Pat. No. 5,559,816 (Basting und Kleinschmidt). Therein, two polarization coupled resonators are proposed with the main light generation taking place in a primary resonator and the wavelength selection being performed by means of a etalon, which is arranged in a secondary resonator with relatively low radiation power. The U.S. Pat. No. 5,835,520 (Palash et al.) and U.S. Pat. No. 5,150,370 (Furuya et al.) also describe arrangements in which the wavelength selective units are positioned on one side of the laser active medium.
The U.S. Pat. Nos. 5,852,627 and 5,856,991 (Ershov) as well as U.S. Pat. No. 5,559,816 (Basting et al.) open the possibility for arranging wavelength selective elements on both sides of the active medium.
The object of the invention is to improve with simple and reliable means a laser with respect to the band width of the radiation emitted as well as the spectral purity of same.
In accordance with the invention this object is achieved by a wavelength selective outcoupling device in the shape of a prism or the like.
According to a first variant a prism in a more specific sense is arranged so that the laser radiation emitted from the resonator impignes (especially perpendicularly) on one side of an isosceles prism, is reflected on the base surface and emerges on the other isosceles side of the prism(especially rectangularly to the surface). Thereby reflections with degrees of reflectance of a view percent, e. g. 4%, (so-called Fresnel reflections) occur on the uncoated surfaces of the isosceles prism resulting in a so-called two-beam-interference with a determined free spectral range and, therefore, a reduced band width and an improved spectral purity.
It is not required for the xe2x80x9cprismxe2x80x9d in the meaning of the present invention to be in a more specific sense a massive body of glass with a triangular cross section. According to a variant of the invention the wavelength selective outcoupling device shows two transparent plates arranged so that they are angled towards one another like a prism, whereby the passing radiation is reflected on a mirror arranged in the radiation path between the plates.
According to a further variant of the invention the wavelength sensitive outcoupling device shows an angular mirror with a plate arranged in front of it, whereby two interfering beams occur by means of a partial reflection on the plate.
Preferred embodiments of the laser according to the invention are described in the dependant claims.
The invention is based on the principle that dual beam or multiple beam interferences are generated by an angular arrangement of partially reflective surfaces and mirrors, which react on the laser resonator so that both, a wavelength selection and an improvement of the spectral purity are achieved.
The prism like arrangement of the elements as provided by the invention represents a xe2x80x9cgeneralizedxe2x80x9d structure for the generation of two-beam or multiple beam interferences. The third reflective surface, additionally arranged in the radiation path according to the invention, offers (other than an etalon) the possibility of wavelength-tuning due to additional physical effects, such as, for example, phase variation of the reflectivity by means of controlled total reflection.
The solutions as provided by the invention further offer the advantage, that in the prism like arrangement the two reflective surfaces which cause the dual beam or multiple beam interferences are not moved themselves. With an etalon (U.S. Pat. No. 5,856,991) it is required to move at least one etalon mirror for the wavelength tuning.
The invention is especially suitable to be realized with an excimer laser, for example, a KrF excimer laser or a F2 laser.